Dwarf Fortress is completely free. The developer does appreciate donations. He states: "As of June 2007, donations are the only money I receive from month to month and are therefore deeply appreciated!"

Difficulty

Default tileset.

Graphics set example.

The game has a very steep learning curve, partly due to its CP437 graphics (code page 437, the character set of the original IBM PC), but also due to the fact that it is one of the most complex games ever released. That said, the curve can be overcome with time, documentation from this wiki, help from people on the forums, and plenty of Beer.

An important part of Dwarf Fortress is that it creates a randomly generated, persistent world for you to play in. (The world is only "on" when you are actually playing in it, however.)

When you die in either the adventure or fortress modes, your next game will be in the same world, albeit a few days later. You can visit your old fortress, or get revenge on the monster that killed your adventurer. If your fortress or your adventurer had done anything of particular note, there's a chance that the game will generate a legend for them, which you can read about in the legends section.

History

The first alpha version of Dwarf Fortress was released on the 8th of August 2006 after nearly 4 years of development (which began in October of 2002). Note that while Dwarf Fortress is technically still in the alpha stage of development, it is fully playable, and definitely enjoyable.

There are some historical milestone versions of DF which you might hear of and wonder about:

0.23.130.23a, the final "2D" version, most often recognized for being the origin of the legendary saga of Boatmurdered (which actually began in version 0.22.110.22f, predating the addition of dyes and proper temperature handling, but switched to newer versions as they were released). It was very different from the current version - for example, fortress mode had no z-axis and did not permit diagonal movement (adventurer mode supported both of these), and as you dug deeper into the mountain (always to the right) you always encountered an underground river, a chasm, a magma river, hidden fun stuff, and finally adamantine. The critters were also very mean back then, especially elephants.

0.28.181.40d, most often referred to as "40d" (which only indicated the number of "bloat" elements implemented and the number of bugfix builds), was current up to April 2010. It didn't have guaranteed magma, had much less interesting underground stuff, the military was much simpler. 0.31 took a long time, so a lot of people are familiar with 40d and there's much wiki-text written on it.

0.31.25 introduced the concept of burrows, an immensely revamped military and healthcare system, the cavern layers as well as a significant number of new animals.

0.34.11 encompasses the Caravan arc and introduced vampires and werewolves into fortress mode as well as necromancers, undead sieges and evil weather.

0.44.07 is the current version. It introduces "world activation" (the world outside your fort continues to progress as you play), multi-tile trees, climbing/jumping, etc.

If you are interested in learning more about the history of the development of the game you can look at the development log.